Q-Switched pulsed waveguide CO2 lasers are commonly used in material processing operations. Many of these operation require laser-radiation pulses delivered by the lasers to have high peak power, for example about 25 Kilowatts (KW) or greater, with peak instantaneous power intensity of several Megawatts per square centimeter (MW/cm2). Such a high peak-power intensity can rapidly cause damage to optical components inserted within the laser cavity, in particular to those components configured to perform the Q-switching function. A consequence of this is that laser operational time may be limited to between 100 to 1000 hours before one or more components need to be replaced. Another consequence of this is that damage to components limits the reliable power output of a laser rather than other aspects of the laser, such as resonator design, cooling arrangements or the like.
FIG. 1A and FIG. 1B schematically illustrate a prior-art, Q-switched, pulsed, CO2 laser 20 including a laser resonator 22. Resonator 22 is terminated at one end thereof by a mirror 24 having a maximally reflecting coating 26, for example, a coating having a reflectivity of about 99.9% or greater. Resonator 22 is terminated at the opposite end thereof by a mirror 28 having a partially reflecting and partially transmitting coating 30, for example, a coating having a transmissivity of about 50% and a reflectivity of about 50%.
Included in resonator 22 is an arrangement 32 including the CO2 gain-medium. Typically such an arrangement would comprise a ceramic slab including a zigzag array of channels or waveguides (not shown) for containing the gain-medium and fold mirrors (not shown) to direct laser-radiation through the channels. A detailed description of such a gain-medium arrangement is not necessary for understanding principles of the present invention. Accordingly, such a detailed description is not presented herein. A detailed description of a zigzag (folded) waveguide arrangement is provided in U.S. Pat. No. 6,192,061, the complete disclosure of which of hereby incorporated by reference. In the description presented below, this gain-medium arrangement is referred to simply as gain-medium 32.
In the gain-medium 32 as depicted in FIGS. 1A and 1B, the waveguides lie in the plane of the illustration, with electrodes (not shown) on opposite sides of the waveguides in planes parallel thereto. A radio-frequency (RF) potential is applied to one of the electrodes as depicted schematically in FIGS. 1A and 1B by a lead 34 and a terminal 36. The other electrode is typically grounded. An RF generator or source for providing the RF generator is not explicitly shown but is adequately represented by the symbol RF in FIGS. 1A and 1B and in other drawings of embodiments of the present invention discussed hereinbelow. Applying the RF potential energizes the gain-medium and generates laser-radiation. In this gain-medium arrangement, laser-radiation generated by the gain-medium is plane polarized, with the electric vector thereof in a plane parallel to the electrodes as illustrated by arrows PP. The direction of travel of radiation in the resonator is indicated by horizontal arrows.
Included in laser resonator 22 is a prior-art Q-switch arrangement 38. Q-switch arrangement 38 includes a thin film polarizer 40, an electro-optical (E-O) switch 42, and a (45-degree) polarization rotator (quarter-wave plate or quarter-wave phase retarder) 44. E-O switch 42 includes an active element 46, usually in the form a crystal of cadmium telluride (CdTe). Crystal 46 is arranged with its optical axis at forty-five degrees to the orientation of PP polarization. A high DC voltage (HV) can be applied to CdTe crystal 46 via electrodes 48 and 49 when a switch 50 is closed. Switch 50 is depicted in an open condition in FIG. 1A, and in a closed condition in FIG. 1B. It should be noted, here, that switch 50, in practice, is an electrical component assembly arranged for pulse switching, but is depicted as a conventional single pole switch in FIGS. 1A and 1B for simplicity of illustration. A detailed description of such a pulse switching component assembly is provided further hereinbelow. End surfaces 46A of crystal 46, through which laser-radiation enters and leaves the crystal, are protected by zinc selenide (ZnSe) windows 52 held in thermal and effectively in optical contact therewith by clamps (not shown). Reflection from exposed surfaces of windows 52 is reduced by antireflection coatings 54. When switch 50 is closed, the high voltage is applied across electrodes 48 and 49, which causes crystal 46 to act as a quarter-wave polarization rotator. When there is no voltage across the crystal (switch 50 open) the polarization of radiation passing therethrough is unchanged.
The purpose of windows 52 is to protect entrance and exit surfaces from damage due to high intensity laser-radiation circulating in the resonator. The widows are clamped against the CdTe crystal, in thermal contact therewith by clamps (not shown) and such that any space between a window and the crystal is less than interference thickness. This reduces reflection losses at the interface therebetween to about the Fresnel reflection loss at an interface between a medium having the refractive index of ZnSe and a medium having the refractive index of CdTe. Additionally, as ZnSe has a much higher thermal conduction coefficient than CdTe, heat generated in the crystal is conducted away from the interface by the ZnSe window, thereby reducing damage at the CdTe crystal surfaces. A detailed description of an E-O switch such as switch 42 is provided in U.S. Pat. No. 5,680,412, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated by reference.
As is well known in the art, the function of Q-switch 38 is to restrict or inhibit circulation of laser-radiation through gain-medium 32 until the gain-medium is sufficiently energized to provide a radiation pulse of the desired power, and then allow radiation to circulate through the medium and build up in intensity, thereby releasing the output pulse through partially transmitting mirror 28. The manner in which this is accomplished by Q-switch 38 is described below with continuing reference to FIGS. 1A and 1B.
Referring first to FIG. 1A, wherein switch 50 is open, laser-radiation polarized in orientation PP leaving gain-medium 32 is transmitted through thin film polarizer 40 and through E-O switch 38 with polarization unchanged. The radiation then passes through quarter-wave phase retarder 44, which converts the plane-polarized radiation to circularly polarized radiation PC. The circularly polarized radiation is reflected from mirror 26 and the direction of circularity is reversed as indicated by arrow PC′. Circularly polarized radiation PC′ then passes through quarter-wave polarization rotator 44 which converts the circularly polarized radiation to plane polarized radiation with an orientation (electric vector) perpendicular to that of radiation PP, as indicated by arrow-tip PS. Radiation PS passes through E-O switch 38 with polarization unchanged. The radiation is reflected by thin film polarizer 40 out of resonator 22. Accordingly, it is not possible for radiation to circulate through energized gain-medium 32 and build in intensity.
Referring next to FIG. 1B, after a predetermined time has been allowed to energize gain-medium 38, switch 50 is closed, causing crystal 46 to act as a polarization rotator as discussed above. As a consequence of this, plane polarized radiation PP passing through the crystal is now converted to circularly polarized radiation PC. Circularly polarized radiation PC passes through polarization rotator 44 and is converted to plane-polarized radiation PS. Plane-polarized radiation PS is reflected from mirror 24 through polarization rotator 44 and is converted thereby to circularly polarized radiation PC. The circularly polarized radiation PC is then converted by crystal 46 to plane-polarized radiation PP. The plane polarized radiation PP is transmitted by thin film polarizer 40 and passes through gain-medium 32. A fraction of the radiation is transmitted by mirror 28 and the remainder is reflected by mirror 28 back through energized gain-medium 32, building in intensity as a result. The intensified radiation then undergoes the aforementioned sequence of polarization changes and returns again to mirror 28.
In this way, laser-radiation is released through partially transmissive mirror 28, initially, as an intense radiation pulse of relatively short duration, for example about 150 nanoseconds (ns). If switch 38 remains closed, the power of the pulse then decays gradually toward some continuous wave (CW) level, which may be several orders of magnitude less than the peak power. In order to generate another laser-radiation pulse, switch 50 must be opened to prevent circulation of radiation as described above, thereby allowing the gain-medium to be reenergized.
In prior-art such lasers, typically, RF power is applied to gain-medium 32 continuously. Laser energy will not be delivered until switch 50 is closed. In many applications of such lasers, laser-radiation pulses are delivered in sequences (“trains” or “bursts”) of between about two and ten or more pulses, with the time interval between pulses being ten or more times longer than the duration of an individual pulse. The pulse-repetition frequency (PRF) of individual pulses in a burst may be between about thirty kilohertz (30 KHz) and 100 KHz. Bursts of pulses may be repeated at a frequency of 1 KHz or greater.
Those skilled in the art will recognize without further illustration that quarter-wave phase-retarder 44, (here transmissive) may be replaced with a reflective phase retarder (RPR) arranged at an angle to incident radiation, with mirror 24 being correspondingly arranged to receive radiation reflected from the RPR and reflect that radiation back to the RPR along its incident path. Those skilled in the art will also recognize that mirror 28 may be replaced by a fully reflective mirror and laser-radiation delivered from the resonator, after a predetermined circulation time therein, by reclosing switch 50, thereby causing the radiation built up in the resonator to be reflected out of the resonator by thin film polarizer 40 in a PS polarization orientation. This is usually termed a “cavity-dumped” mode of operation.
Whatever the phase retarder arrangement or operation mode, components of Q-switch arrangement 38 are prone to optical damage by radiation build up in the laser resonator. Typically, phase retarder 44 (or a reflective equivalent) is the most likely or the first component to be damaged. Antireflection coatings 54 are the next most likely, or the next components to be damaged. It is an object of the present invention to eliminate one or more of these components and preferably to protect any remaining components from optical damage.